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Viruses :

Figure 27.2 The three domains of life :

Figure 27.2 The three domains of life

Table 27.2 A Comparison of the Three Domains of Life :

Table 27.2 A Comparison of the Three Domains of Life

Viruses (Latin, poison) :

Viruses (Latin, poison) Are noncellular
Size 20-200 nm
Always have at least two parts:
--an outer capsid composed of
protein subunits
--an inner core of nucleic acid
DNA or RNA
May be surrounded by membranous envelope
May also contain various proteins, especially enzymes such as polymerases

Viruses :

Are called obligate intracellular parasites, which means they
cannot live outside of a living cell
Each kind of virus usually infects one kind of organism and/or one kind of tissue,
e.g. hepatitis virus only infects liver cells
Believed that viruses are derived from the kind of cell they infect and must have evolved after cells
They can mutate and, therefore, they evolve; e.g. flu viruses are constantly mutating Viruses Figure 18.x4 Hepatitis

Reproduction of HIV-1 :

Reproduction of HIV-1

Infectious Agents Smaller than a Virus :

Infectious Agents Smaller than a Virus Viroids
naked circular strands of RNA several hundred nucleotides long that infect plants
replicate in host cells using cellular enzymes
symptoms include abnormal development and stunted growth in the plants
one viroid disease has killed over 10 million coconut palms in the Philippines

Infectious Agents Smaller than a Virus :

Prions
protein particles that can cause other proteins in the cell to have a misshapen tertiary structure
cause a number of degenerative brain diseases, including scrapie in sheep, mad cow disease, and Creutzfeldt-Jakob disease in humans Infectious Agents Smaller than a Virus